FIG. 1 schematically shows prior art image recording apparatus 20 with light-emitting diode (LED) printbar 21. It should be appreciated that an LED printbar may also be referred to as an LED printhead (LPH). Printbar 21 is an example of an LED full width array imager. An LED full width array imager consists of an arrangement of a large number of closely spaced LEDs in a linear array. By providing relative motion between the LED printbar and a photoreceptor in a process direction, and by selectively energizing the LEDs at the proper times in a scan direction, a desired latent electrostatic image can be produced on the recording member. The production of a desired latent image is usually performed by having each LED expose a corresponding pixel on the recording member in accordance with image-defining video data information applied to the printbar through driver circuitry. Conventionally, digital data signals from a data source, which may be a Raster Input Scanner (RIS), a computer, a word processor or some other source of digitized image data is clocked into a shift register. Some time after the start of a line signal, individual LED drive circuits are then selectively energized to control the on/off timing of currents flowing through the LEDs. The LEDs selectively turn on and off at fixed intervals to form a line exposure pattern on the surface of the photoreceptor. A complete image is formed by successive line exposures.
The following provides further detail regarding prior art apparatus 20. Printbar 21 includes: LED's controlled according to recording signals supplied from an unrepresented external device; a rotary drum 22 provided with a photoreceptor along the periphery thereof; a rod lens array 23 for focusing the light beams of the LEDs in the printing head 21 onto the photoreceptor surface of the drum 22; a corona charger 24 for charging the photoreceptor in advance; a developing station 25 for developing an electrostatic latent image with toner; a recording sheet 26; a cassette 27 housing a plurality of recording sheets 26; a feed roller 28 for feeding the recording sheet 26 from the cassette 27; registration rollers 29 for matching the front end of the recording sheet with the leading end of the image formed on the drum 22; a transfer charger 30 for transferring the developed image from the drum 22 onto the recording sheet 26; a separating roller 31 for separating the recording sheet from the drum 22; a belt 32 for transporting the recording sheet; fixing rollers 33; discharge rollers 34 for discharging the recording sheet onto a tray 35; a blade cleaner 36 for removing the toner remaining on the drum 22; a container 37 for the recovered toner; and a lamp 38 for eliminating charge remaining on the drum 22.
An LED print-head (LPH), e.g., LPH 50, is built by assembling a number of LED array chips end-to-end, e.g., LED array chips 521 through 5240. For example, a 1200 dots per inch (dpi) A3 sized print-head could be constructed by assembling forty (40) individual LED array chips each having three hundred eighty four (384) LEDs, or fifteen thousand three hundred sixty (15,360) total LEDs across the entire LPH, as shown in FIG. 2A. Typically this type of print-head is driven with a ⅛th matrix drive circuit, e.g., matrix drive circuits 541 through 5448, (See U.S. Pat. No. 6,172,701) and has an LED arrangement as shown in FIG. 2B through 2D. It should be appreciated that the foregoing arrangement includes a single shift register, i.e., shift register 56, which loads print data into the entire LPH and results in all LEDs within the LPH firing within eight actuations of the strobe. Thus, for example, LED 58a of LED array chip 521 simultaneously fires as all other LEDs 58a on all other LED array chips 522 through 5240 fire. Similarly, all LEDs 58b, 58c, 58d, 58e, 58f, 58g, 58h collectively as groups simultaneously fire on all LED array chips 521 through 5240.
Ideally, the process direction profile of the LEDs of the print-head would be flat, i.e., there is no process direction profile in the scan direction; however, this arrangement does not occur as it is not possible to manufacture LED array chips in such a fashion. It should be appreciated that as used herein, the process direction is represented by bi-directional arrow 59a while the scan direction is represented by bi-directional arrow 59b. As array chips 521 through 5240 are positioned end-to-end, a true straight line is not formed thereby resulting in process direction profile errors. The measured process direction profile of a typical print-head of the type shown in FIG. 2A is provided in FIG. 3A. It has a process direction profile range of 106 μm (+80 μm to −26 μm) across the length of the LPH. Alternately, a chip averaged process direction profile of FIG. 3A, is shown in FIG. 3B, and has a range of 88 μm (+69 μm to −19 μm). It should be appreciated that the “chip average” is the average process direction position of all LEDs within a discreet LED array chip. A typical specification range for the measured process direction profile is 100 μm, which is approximately equal to five lines of correction for a 1200 dpi×1200 dpi printing system.
For a 1200 dpi×1200 dpi printing system, the process direction profile range of a typical LPH represents a misalignment of about five scan lines in a printed image, while for a 1200 dpi×2400 dpi system it represents a misalignment of about eight scan-lines. It should be appreciated that the foregoing misalignments are based on the physical location of the LEDs and are therefore present in a printed image if a method of process direction profile compensation is not implemented. Moreover, the misalignment is further increased based on the fact that between the time that the first LED on a chip is activated and the last LED on that chip is activated, the print media, e.g., toner drum, sheet, image bearing belt, etc., has traveled in the process direction a particular distance based on the media's travel speed.
Thus, the process direction profile of an LED print-head impacts the image quality of a printer unless a method of compensation is implemented with the print controller. Although use of the print controller improves image quality, such use is quite complex and results in significant processing overhead which may then impact print speed. In known systems, the print controller variably delays the print data in the scan direction in order to minimize the misalignment, i.e., to compensate for the LPH process direction profile. For a ⅛th matrix driven LPH with print controller process direction profile compensation, the effective process direction profile range can be reduced to approximately one scan-line. Although this is a significant improvement, the improvement is provided at the expense of processing overhead and complexity.
The present disclosure addresses a system and method for compensating for LED print-head process direction profile variability.